1. Field of the Invention
This invention relates to a video endoscope system that enables an object to be observed through autofluorescence caused from a living body.
2. Description of the Related Art
Video endoscope system for obtaining a color image of an object under examination, for example a coelomic wall of the living body and the like, are known and being utilized. RGB frame sequential system is also known as method for obtaining a color image of an object under examination. The RGB frame sequential system is adapted to obtain a color video signal by synthetically combining monochromatic video signals obtained separately while the object under examination is being irradiated with blue, green and red light, respectively.
Besides, video endoscope systems enable a living body to be observed through fluorescence (i.e., autofluorescence) generated from the living body when the living body is irradiated with excitation light. The autofluorescence generated from diseased tissue is weaker than the autofluorescence generated from healthy tissue. Therefore, an operator can explore the object under examination through the autofluorescence generated from the object and recognize an area where the autofluorescence is weak as a diseased area.
Recently, video endoscope systems that are adopted with the RGB frame sequential system and incorporated with the functional feature of fluorescence observation have been proposed. Such a video endoscope system can display both an ordinary moving color image of an object under examination and a moving fluorescent image of the object based on the autofluorescence generated from the object. Therefore, the operator using the video endoscope system can selectively acquire either the ordinary moving color image of the object or the moving fluorescent image of the object based on the autofluorescence generated from of the object. Such a video endoscope system has a light source unit for emitting light with which the object is irradiated, and a CCD for picking up an image of the object that is illuminated with the light. When the video endoscope system is operating in the ordinary observation mode, the light source unit emits blue, green and red light sequentially and repeatedly. When, on the other hand, the video endoscope apparatus is operating in the fluorescence observation mode, the light source unit emits excitation light and white light alternately and repeatedly.
FIG. 19 is a timing chart for illumination of light emitted from the light source unit and processes of image acquirement by the CCD. Firstly, the operation of the video endoscope system in the ordinary observation mode will be described by referring to FIG. 19A and FIG. 19B. FIG. 19A shows the operation of the CCD in the ordinary observation mode and FIG. 19B shows the periods in which illumination light emitted from the light source unit is irradiated in the ordinary observation mode. A xe2x80x9cB irradiationxe2x80x9d period during which blue light is emitted from the light source unit corresponds to a xe2x80x9cB accumulationxe2x80x9d period for the CCD, which means that an electric charge corresponding to the image of the object formed from blue light is accumulated in each pixel of the CCD when the object under examination is irradiated with blue light. The electric charge accumulated in the xe2x80x9cB accumulationxe2x80x9d period is output as B video signal in a xe2x80x9cB transferxe2x80x9d period that comes immediately after the xe2x80x9cB accumulationxe2x80x9d period. The xe2x80x9cG accumulationxe2x80x9d period that comes immediately after the xe2x80x9cB transferxe2x80x9d period corresponds to a xe2x80x9cG irradiationxe2x80x9d period during which green light is emitted from the light source unit, which means that an electric charge corresponding to the image of the object formed from green light is accumulated in each pixel of the CCD during the xe2x80x9cG accumulationxe2x80x9d period. The electric charge accumulated in the xe2x80x9cG accumulated period is output as G video signal in a xe2x80x9cG transferxe2x80x9d period that comes immediately after the xe2x80x9cG accumulationxe2x80x9d period. The xe2x80x9cR accumulationxe2x80x9d period that comes immediately after the xe2x80x9cG transferxe2x80x9d period corresponds to an xe2x80x9cR irradiationxe2x80x9d period during which red light is emitted from the light source unit, which means that an electric charge corresponding to the image of the object formed from red light is accumulated in each pixel of the CCD during the xe2x80x9cR accumulationxe2x80x9d period. The electric charge accumulated in the xe2x80x9cR accumulationxe2x80x9d period is output as R video signal in an xe2x80x9cR transferxe2x80x9d period that comes immediately after the xe2x80x9cR accumulationxe2x80x9d period. Then, a color video signal representing a color image of the object under examination is synthesized from the B video signal, the G video signal and the R video signal output sequentially from the CCD.
Next, the operation of the video endoscope system in the fluorescence observation mode will be described with reference to FIG. 19C and FIG. 19D. FIG. 19C shows the operation of the CCD in the fluorescence observation mode and FIG. 19D shows the periods in which illumination light emitted from the light source unit is irradiated in the fluorescence observation mode. The object under examination generates autofluorescence as it is irradiated with excitation light (ultra violet light). Then, the CCD picks up the image formed from the autofluorescence generated from the object. Thus, a xe2x80x9cUV irradiationxe2x80x9d period during which the excitation light (ultra violet light) is emitted from the light source unit corresponds to an xe2x80x9cF accumulationxe2x80x9d period for the CCD, which means that an electric charge corresponding to the image of the object formed from the autofluorescence generated from the object is accumulated in each pixel of the CCD when the object under examination is irradiated with the excitation light. The electric charge accumulated in the xe2x80x9cF accumulationxe2x80x9d period is output as F video signal in an xe2x80x9cF transferxe2x80x9d period that comes immediately after the xe2x80x9cF accumulationxe2x80x9d period. A xe2x80x9cW irradiationxe2x80x9d period during which white light is emitted from the light source unit corresponds to a xe2x80x9cW accumulationxe2x80x9d period of the CCD, which means that an electric charge corresponding to the image of the object formed from the white light is accumulated in each pixel of the CCD when the object under examination is irradiated with the white light. The electric charge accumulated in the xe2x80x9cW accumulationxe2x80x9d period is output as W video signal in a xe2x80x9cW transferxe2x80x9d period that comes immediately after the xe2x80x9cW accumulationxe2x80x9d period. A video signal as to the object to be used for diagnosis is synthesized from the F video signal and the W video signal output from the CCD. More specifically, the video signal of the object, to be used for diagnosis is obtained by subtracting the F video signal from the W video signal.
In the above described video endoscope system, the xe2x80x9cW irradiationxe2x80x9d period is as long as the xe2x80x9cUV irradiationxe2x80x9d period, as shown in FIG. 19D. Therefore, the xe2x80x9cW accumulationxe2x80x9d period is as long as the xe2x80x9cF accumulationxe2x80x9d period, as shown in FIG. 19C. Now, the autofluorescence generated from the object is very weak. Therefore, when obtaining a video signal to be used for diagnosis is generated from a W video signal and an F video signal, the F video signal needs to be greatly amplified. However, as the amplification factor is increased, the S/N ratio of the F video signal falls and the video signal to be used for diagnosis which is ultimately obtained may contain a high level of noise.
In view of the above identified circumstances, it is therefore the object of the present invention to provide a video endoscope system which is adapted to obtain an image to be used for diagnosis, without lowering the S/N ratio.
In the first aspect of the present invention, the above object is achieved by a video endoscope system which has an illuminating optical system for illuminating an object under examination, and a light source unit which emits visible light and excitation light for exciting a living tissue of the object to cause fluorescence. The light source unit alternately transmits the visible light and the excitation light to the illuminating optical system so that a period the excitation light is transmitted to the illuminating optical system may be longer than a period the visible light is transmitted to the illuminating optical system. The video endoscope system also has an objective optical system which converges optical components of the light, other than the excitation light, coming from a surface of the object to form an image of the object, and an image pickup device which picks up the image of the object formed by the objective optical system to convert the image into a video signal. The video endoscope system also has a processor which generates a reference video signal, based on a video signal obtained by the image pickup device during a period when the visible light is transmitted to the illuminating optical system, and a fluorescence video signal based on a video signal obtained by the image pickup device during a period when the excitation light is transmitted to the illuminating optical system.
With the above arrangement, the period during which the object under examination is irradiated with the excitation light is made longer than the period during which the object is illuminated with the visible light. Therefore, the period during which electric charges attributable to the autofluorescence generated from the object accumulates in the image pickup device such as a CCD is made longer than the period during which electric charges attributable to the visible light accumulates in the image pickup device. Thus, the intensity of the video signal corresponding to the image formed by the fluorescence generated from the object output from the image pickup device rises to a level comparable to that of the video signal corresponding to the image formed from the visible light. Consequently, it is no longer necessary to excessively amplify the signal. Therefore, as the fluorescence video signal is subtracted from the reference video signal, a video signal to be used for diagnosis can be produced without lowering the S/N ratio to clearly show any diseased part.
In the second aspect of the invention, there is provided a video endoscope system which has an illuminating optical system which guides light to an object under examination, and a light source unit which emits visible light and excitation light for exciting a living tissue of the object to cause fluorescence. The light source unit transmits the visible light and excitation light to the illuminating optical system and alternately transmit visible light and excitation light to transmit them to the illuminating optical system. The period when the light source unit transmits the excitation light and the period when the light source unit transmits the visible light are adjustable. The video endoscope system also has an objective optical system which converges the optical components of light other than excitation light coming from the surface of the object to form an image of the object, and an image pickup device which picks up the image of the object formed by the objective optical system to convert it into a video signal. The video endoscope system also has a processor which generates a reference video signal based on a video signal obtained by the image pickup device during a period when the visible light is transmitted to the illuminating optical system, and a fluorescence video signal based on a video signal obtained by the image pickup device during a period when the excitation light is transmitted to the illuminating optical system.
With the above arrangement, the period during which the object under examination is irradiated with the excitation light and the period during which the object is illuminated with the visible light can be changed so that the intensity level of the reference video signal and that of the fluorescence video signal may be appropriately regulated.
The light source unit may include a visible light source for emitting visible light and an excitation light source for emitting excitation light. If such is the case, the light source unit may alternately block excitation light and visible light. Typically, this is done by means of a light blocking member, such as a rotary shutter.
Alternatively, the light source unit may be composed of a single light source for emitting light containing a frequency band of both visible light and excitation light. Then, the light source unit alternately transmits visible light and excitation light to the illuminating optical system by alternately inserting a filter transmitting only visible light and a filter transmitting only excitation light in the optical path of light emitted from the light source.